Reparent Mouth Rig to Different Character's Armature?
I've discovered a way to do this without having to weight paint again.


In the Outliner Window, drag the mesh for your mouth to the other armature and set the Object as the parent. You'll see that the mouth mesh is still weight-painted to the old armature, but you won't be able to move it in pose mode (yet)
Make sure there aren't any constraints added to the armature from the mouth that you're trying to join to the new armature.
In Object Mode, highlight the mouth armature followed by the armature you want to join it with and hit CTRL+J to join them.
Select the bone parent in edit mode for the mouth bones you transferred over under the armature tab.
In Object Mode select the mouth and Shift-select the new armature and type CTRL+P and parent the mouth mesh to the new armature with empty groups.


That's it. You should now see your mouth has the same weight painting it once had but in a new armature! However, you may have to go into edit mode and translate it a bit to get it to line up properly with the bones.